Fuel cells have recently attracted attention as effective solutions to environmental and energy problems. A fuel cell oxidizes a fuel such as hydrogen using an oxidant such as oxygen and converts the resulting chemical energy into electrical energy.
Fuel cells are classified into alkaline type, phosphoric acid type, polymer electrolyte type, molten carbonate type, solid oxide type, etc., depending on the kinds of electrolytes.
Polymer electrolyte fuel cells (PEFC) have a low temperature operation and a high power density and thus are capable of miniaturization and lightening, thereby being expected to be applied as portable power supplies, domestic power supplies or vehicle power sources.
Although a perfluorocarbon slfonic acid membrane is used as an electrolyte membrane for polymer electrolyte fuel cells (PEFC), it poses the problem of the environmental load in disposal being high since the membrane contains fluorine.
Hence, a hydrocarbon film not containing fluorine (see Patent Document 1) has been developed. The sulfonic acid group in this hydrocarbon film not containing fluorine shows excellent proton conductivity since the group can dissociate a hydrogen ion.
When power is generated using a polymer electrolyte fuel cell, humidified hydrogen is fed to an anode and humidified oxygen to a cathode in an MEA in order to enhance its power generation performance. Therefore, a humidifier is needed for a fuel cell system, whereby a polymer electrolyte fuel cell that does not need a humidifier, or a low or no humidity MEA is desired for cost effectiveness and compactification.
However, for conventional hydrocarbon films, sulfonic acid groups (SO3H groups) are at random positions as shown in a conceptual diagram in FIG. 4, so that a passage for protons can hardly be made and thus there is the problem that the proton conductivity tends to be readily low particularly in low humidity conditions.
A hydrocarbon film having hydrophilic and hydrophobic sites controlled is proposed for solving this problem (see Patent Document 2). In this hydrocarbon film, a passage for protons made by hydrophilic sites is formed by phase separation as shown in a conceptual diagram of FIG. 5, so it is still insufficient although the film exhibits relatively high proton conductivity even in low humidity conditions.
On the other hand, in a liquid crystalline state of a sulfonated liquid crystalline monomer material having a smectic phase, ionic conduction sites of the sulfonic acid groups are found to be laid on top of another densely. In addition, it is proposed that a solid state of maintaining a molecular arrangement of the liquid crystalline state exhibit excellent proton conductivity since a more controlled passage for protons is formed as illustrated in a conceptual diagram of FIG. 6 (see Patent Document 3).
However, there is a problem in that sulfonated liquid crystalline monomer material does not have sufficient mechanical characteristics and thus cannot be used as a membrane.
Hence, a sulfonated liquid crystalline polymer is considered to be used in place of sulfonated liquid crystalline monomer material. Incidentally, if the polymer can be made a solid state while maintaining a molecular arrangement of a liquid crystalline state, as shown in FIG. 7, the membrane exhibits high proton conductivity even in low humidity or no humidity conditions since a membrane in which a more controlled passage for protons is formed can be obtained.
In this case, the disordered molecular arrangement in the solid state is made to be phase-transferred to a liquid crystalline state once, and the disordered molecular arrangement is put in order, and then cooled to obtain a solid in which a passage for protons is formed. However, the sulfonated liquid crystalline polymer shown in FIG. 7 is still insufficient in mechanical characteristics and is difficult to retain as a membrane.    Patent Document 1: Japanese Patent Laid-Open No. 2006-179448    Patent Document 2: Japanese Patent Laid-Open No. 2005-194517    Patent Document 3: Japanese Patent Laid-Open No. 2003-55337